In conventional image processing systems, a raster scan video camera is provided for use with raster scan image processing techniques. Examples of these are conventional vidicon cameras and two-dimensional CCD sensing arrays, each of which is designed for predetermined sequential scanning of the pixels in the rows and columns which constitute the lines of the photosensitive surface. Sequential scanning provides sequential data, with the result that an entire frame must be scanned one line at a time before the next frame begins.
In many applications, the image consists of a lot of useless information. For example, in computer vision systems for industrial control applications, the edges of a single object on a conveyor belt must be traced by an identifying and positioning system, and all that is necessary is a small neighborhood of pixels around the edges of the object. A high speed target tracking system is another example. In the latter, due to the sequential nature of the video camera operation, much time is wasted scanning and processing the background image, while this time could be better spent rescanning the target itself, which is moving in space between successive frames.
In previous and related Israel patent application No. 79485 filed July 21, 1986, commonly owned by the owners of the present application, there was described a random scan imaging system including a coordinated image processor capable of scanning the image and processing it in an order that is not defined in advance. The system relies on a number of stored scan patterns which are translated into horizontal and vertical deflection signals for the electron beam of a vidicon TV camera. The processing algorithms, specific to the application, define the paths of scanned pixels either statistically or even in a dynamic manner, thereby eliminating redundant scanning.
Miniaturization techniques would make two-dimensional CCD sensing arrays (such as Fairchild CCD222) attractive as a replacement for the vidicon camera in the random scan imaging system application, because of their higher reliability, lower cost, lower supply voltage and power requirements, and reduced sensitivity to mechanical vibration. However, the applicability of these types of sensors is limited by the basic chip architecture which is designed and built for sequential, not random scanning. That is, since CCD arrays are basically a series of shift registers each of which incorporates a series of photodetector cells, the data must be shifted out sequentially, one data value at a time. As mentioned earlier, this is time inefficient not to mention the tremendous amount of hardware required by the image preprocessor to handle all of the pixel data, much of which is irrelevant.
For example, if image resolution of 1000.times.1000 is desired over a wide field, conventional image processing techniques would call for an image sensor design utilizing 1 million pixels. If each pixel has just one bit of information, there are 2 to the power 10 to the 6th combinations that can exist, which is an enormous and unwieldy number. Several image processing techniques have been proposed for sorting pixel data and attempting to improve the efficiency with which image preprocessors operate.
In one case, a processor chip architecture has been discussed which allows a group of regional pixels to provide a neighbordhood transform containing data about regions of the input image, as described in the paper by Riesenbach et al entitled, "A VLSI Architecture for Real Time Image Processing", published in the proceedings of the IEEE International Conference on Computer Design, October 1986. However, these and other processing techniques require sequential raster scanning and do not eliminate the overall hardware and time constraints outlined above.
There is also known in the prior art an image sensor using a dynamic random access memory, as disclosed in U.S. Pat. No. 4,441,125, issued April 3, 1984 to Parkinson. This device is based on a modification to a commercially available random access memory chip. Normal RAM construction uses a light-tight opaque package to restrict light from impinging on the memory cells. By virtue of the modification, light is directed via lenses to impinge on the memory cells and alter their digital data states which are representative of the light image. However, since the data states are digital, this approach does not provide information about the absolute level of the light intensity, rather about a threshold level only. Further, the patent does not disclose a random access technique of scanning, relying on the sequential raster scan mode instead.
A random scan image sensor has been proposed in the prior art in a paper entitled "Charge-Injection Imaging: Operating Techniques and Performance Characteristics", by H. K. Burke and G. J. Michon, published in the IEEE Transactions on Electron Devices, volume ED-23, pages 189-195, February 1976. The charge injection device (CID) consists of two neighboring MOS capacitors. Photo-generated charge accumulates in the MOS inversion region under the charged electrodes. When the voltage is set to zero on both electrodes, the accumulated charge is no longer held in place and is "injected" into the substrate, thus resetting the device in preparation for successive accumulation.
Since the functions of charge accumulation and memory are performed by the same device, the light sampling and readout processes are coupled together very tightly. This introduces problems in the accuracy of the device, since the readout process does not suspend the sampling process, and light continues to induce charge even after the end of the sampling period.
Therefore, it would be desirable to provide an intelligent scan image sensor designed to fully exploit the capabilities of a random scan imaging system in eliminating irrevelant image information and achieving greater efficiency in the operation of image preprocessor algorithms.